There are presently a wide variety of eyewear styles, including sunglasses, that are commercially available. These sunglasses vary both in the style of the frame and the optical characteristics of the lenses mounted in the frame. The need for a wide variety of lens characteristics stems at least in part from the variety of circumstances under which sunglasses are used. For example, blue-tinted lenses are often considered superior for boating and other water-related activities. Neutral-density lenses are considered superior where it is undesirable for the sunglass lenses to change the perceived color of objects. Personal preference also plays a role in the variety of lens tints that are commercially available. Even for a given lens color, different degrees of optical density (i.e., "opaqueness ") are often desired, depending upon lighting conditions and personal preference. For example, a higher optical density may be desired for midday conditions, particularly in highly reflective environments such as snow, while a lower optical density may be preferred later in the day, particularly in less reflective environments. As a result, there is a need for a sunglass lens to have a wide variety of optical properties.
In the past, it has been difficult for users to select the optimal lens for the wide variety of conditions in which sunglasses are worn. Not only is there an often prohibitive expense in purchasing sunglasses for a variety of conditions, but there is also the inconvenience of carrying around different sunglasses for different conditions which may vary during a single day. For example, for a skier to have the optimum lens on a partly cloudy day, it would be necessary for the skier to carry at least two pairs of sunglasses having lenses with different optical densities. As a result of the costs and inconvenience of having different sunglasses available for different conditions, most sunglass wearers do not have the optimum lens much of the time. Instead, the sunglass wearer must compromise on a lens that, while not optimum for any lighting condition or activity, is at least somewhat satisfactory for all lighting conditions and activities.
It would be possible for sunglasses to have a wide variety of optical characteristics if the lenses could be easily and quickly removed from the sunglass frames. The wearer would then only need to purchase and carry a variety of lenses. A suitable selection of lenses would be far more compact and less expensive than a comparable selection of eyeglasses. However, lenses are preferably removed from conventional sunglass frames only by optical professionals. Furthermore, frequent removal and reinstallation of lenses in conventional sunglass frames tend to weaken the frictional connection between a lens and frame so that eventually the lens would become loose enough to fall out during strenuous activity.
As a result of the limitations of conventional sunglasses, there is a need for sunglasses that allow the optical properties of the sunglass lenses to be easily and quickly varied by the wearer in a convenient manner.